1. Field of the Invention
The invention relates to the control of cooling flows for the walls of high-temperature combustion chambers such as are used in turbomachines, and especially turboshaft engines for aircraft.
2. Summary of the Prior Art
The requirements for such a combustion chamber include a very low pressure drop along the combustion chamber walls to ensure minimum impairment of the energy of the cooling fluid and to produce a very uniform peripheral and radial temperature distribution at the turbine inlet downstream from the chamber.
The thermodynamic cycles currently chosen for civil and military turbojet engines result in ever-higher temperatures at the combustion chamber outlet, and these high temperatures help to reduce the specific fuel consumption of the engine.
However, increasing the temperature in the combustion chamber leads to higher Mach numbers, which may rise locally to a value M=0.25. Also, in combustion chambers which are sharply convergent in their downstream zone, and particularly in combustion chambers having two separate heads, the Mach number changes appreciably towards the downstream end of the chamber. This change affects combustion chamber performance and, in particular, the rate of flow of cooling air.
The reason for this is that an increase in Mach numbers in the combustion chamber towards its downstream end reduces the static pressure in the combustion chamber in the direction of gas flow. This causes an increase in the pressure drop along the combustion chamber walls towards the downstream end. However, for a given configuration of the cooling air inlet holes the air flow rate therethrough depends upon the pressure drop and varies in the same sense as the latter.
The pressure drop may almost double in value in the convergent zone, with the result that the rate of cooling air flow may experience a local increase of 30% if the combustion chamber walls are of uniform permeability throughout their length. The accentuation of the radial profile of twin-head combustion chambers increases these pressure drops and cooling flow rate variations.
An increase in the cooling flow rate at the combustion chamber outlet is incompatible with the temperature profiles which are desirable at the combustion chamber outlet. These include the radial temperature factor (F.R.T.) and the local temperature factor (F.L.T.) which characterize the hottest point of the temperature profiles.